A quantitative determination of L-glutamate is very important for process control or product analysis in the field of a food product industry or amino acid industry. A convenient quantitative determination of L-glutamate is also required in the field of clinical chemistry laboratories. For example, the determination is very important for a determination of the activity of glutamate-oxaloacetate transaminase (GOT) or glutamate-pyruvate transaminase (GPT) in a serum collected for a diagnosis of liver function and myocardial infarction.
In the field of the food product industry or amino acid industry, an instrumental analysis using liquid chromatography, especially an amino acid analyzer, is widely used for the quantative determination. It has been recently proposed to use determining reagents using enzymes. These methods are advantageous in easy pretreatment of a sample and accurate determination due to substrate specificity. The enzymes used for the determining reagent include glutamate dehydrogenase, glutamate oxidase, glutamate decarboxylase or the like. The determining reagent using glutamate dehydrogenase is presently commercially available. In the determination of the activity of GOT or GPT, a method wherein oxaloacetic acid or pyruvic acid is analyzed with a determining reagent containing malate dehydrogenase or lactate dehydrogenase is lately adopted. This method is also advantageous in accurate determination and easy pretreatment of a sample. However, in these methods, it takes quite a long time to determine and a life time of the determining reagent is very short.
In order to overcome the above mentioned defects, an enzyme sensor composed of an enzyme having excellent substrate specificity and an electrode has been developed. As for a L-glutamate sensor using enzymes, an enzyme sensor composed of glutamate dehydrogenase and an ammonium ion electrode is proposed. A microorganism sensor composed of an electrode and a microorganism instead of an enzyme is also studied. Presently proposed is a microorganism sensor composed of Escherichia coli and a carbon dioxide gas electrode (see Protein, Nucleic acid and Enzyme, volume 30, No. 4, pp. 245 to 298, 1985, especially Table 2 of pp. 262 and Table 1 of pg. 265). Further, another enzyme sensor composed of glutamate oxidase is known to the art (see Japanese patent publication (unexamined) 34882/1984). However, these sensors have problems in life time and selectivity to a substrate to be measured. In order to improve the defects, another type of enzyme sensor composed of glutamine synthetase isolated from thermophilic bacteria and an ammonia gas electrode is proposed (Denki Kagaku, 54, No. 3 (1986), 291 to 292). This sensor enhances its life time, but it is not sensitive in the region of low concentration of glutamate. It is also desired that the enzyme sensor is miniaturized as far as possible, but it is difficult to miniaturize it when an electrode made of glass is employed.